Pour Guinness into a saucepan and bring to a boil. Once boiling, lower to a simmer and let the liquid reduce until it measures 1 cup. Use a large glass measuring cup for this. Once it looks like the liquid has been reduced by half, you can pour into the measuring cup to check. Pour back into the saucepan and continue cooking, if necessary. Let the reduced Guinness cool completely before using. You can do this step a day or two in advance and store covered in the refrigerator until ready to use.

Preheat the oven to 350°F. Butter and flour three 8-inch round cake pans.

In a large bowl, whisk together both sugars, flour, cocoa powder, baking powder, baking soda, and salt.

Add eggs, buttermilk, oil, vanilla, and ¾ cup reduced Guinness. Mix with an electric mixer on medium speed for 1 to 2 minutes, or until all ingredients are incorporated. Divide batter evenly among the prepared pans.

Bake for 22 to 25 minutes, or until a wooden pick inserted in the center comes out clean. Let the cakes cool in the pan for 10 to 15 minutes, then transfer to a wire rack to cool completely.

Place chocolate in a heat-safe bowl. Heat cream over medium heat in a small saucepan until just simmering around the edges. Pour hot cream over the chocolate and let it sit for 5 minutes. Whisk until smooth. Let the ganache cool completely.

Ganache can be made 1-2 days ahead. Store covered in the refrigerator until ready to use. Let it come to room temperature before using. You may need to heat it in the microwave for 10-15 seconds to return it back to a drizzling consistency.

Place chocolate in a microwave-safe bowl and heat in 30 second intervals, stirring between each, until melted and smooth. Set aside to cool.

Cream butter and shortening on medium-high speed with an electric mixer. Add cocoa powder, vanilla, and stout reduction. Mix well, scraping the sides of the bowl as necessary.

Gradually add powdered sugar, mixing until incorporated. Mix in melted chocolate.